Well well well… perhaps I do not understand what you are trying to do, but why not just roll 1D100 to choose which storylet will run ? In this game, Graham Nelson shows us how to implement such a mechanism (Go here to see the code). With a dice with 100 faces, isn’t it easier to manage % probabilities ? Sorry if my understanding is wrong.
Oops clicked too fast on the reply button, sorry. In the game mentionned above, the author proposes also a method to stock dicerolls for future purposes and to apply modifiers. Seems appropriate to control dependencies and cross-impacts between the rolls and the chance to a storylet to occur.